A number of fields of science and manufacturing require precise positioning of a stage with respect to another stage, a frame, or other frame of reference. One such field is lithography, in particular, as applied to semiconductor wafer fabrication.
In these lithographic systems, a wafer is positioned on a surface, sometimes called a wafer table, which is in turn movable with respect to another surface or frame, sometimes called the wafer stage. The wafer stage may itself be movable. Typically, light, or an electron beam, passes through a mask mounted on a reticle, through a projection lens, and onto the wafer. The beam thereby exposes a pattern on the wafer, as dictated by the mask. Both the reticle and the wafer may be movable, so as to repetitively expose the mask pattern on different portions of the wafer.
An example of such a system is provided in FIG. 1. As shown in that figure, lithographic processing is performed by an exposure apparatus 10. Generally, a pattern of an integrated circuit is transferred from a reticle 32 onto a semiconductor wafer 62. The exposure apparatus 10 is mounted on a base 99, i.e., a floor, base, or some other supporting structure.
At least some of the components of the exposure apparatus 10 are mounted on a frame 12. In some examples, the frame 12 is rigid. The design of the frame 12 can be varied according to the design requirements of the rest of the exposure apparatus 10. Alternatively, a number of different frames or support structures may be employed to suitably position the various components of the exposure apparatus 10. In the example shown in FIG. 1, the reticle assembly 30, which typically includes a reticle stage 34, holds and positions the reticle 32 relative to the lens assembly 50 and the wafer assembly 60, which typically includes a wafer stage 64. Similarly the wafer stage 64 holds and positions the wafer with respect to the projected image of the reticle 32. In the prior art, various devices 14 may be employed to achieve such positioning, including linear and planar motors. The requirements for this positioning may vary with the design requirements of the system.
The illumination system 20 provides a light source for exposure of the wafer. In some examples, the illumination system 20 includes an illumination source 22 and an illumination optical assembly 24. The illumination source 22 emits a beam of light energy. The illumination optical assembly 24 guides the beam of light energy from the illumination source 22 to the lens assembly 50. The beam illuminates selectively different portions of the reticle 32 and exposes the wafer 62. In FIG. 1, the illumination source 22 is supported above the reticle 32. Alternatively, the illumination source is positioned to one side of the of the frame 12, and the optical assembly 24 directs the light energy to the reticle 32.
Each of the components of such a system may require precise positioning. In particular, the mask and/or the wafer must be precisely positioned relative to each other and relative to the lens, so that the mask pattern is exposed on the appropriate portion of the wafer. To achieve such positioning, various components of the system may be adjustable. In particular, the reticle and/or the lens may be adjustable. Further, the wafer stage may be adjustable. A method of extremely fine adjustment is needed to precisely position the components with respect to each other.
Various designs have been proposed to provide such precise positioning. For instance, U.S. Pat. No. 4,506,204 discloses apparatus for electromagnetic alignment using at least three magnet assemblies in spaced relationship, with coil assemblies positioned in the high-flux region of the magnets. By controlling the current flowing through the coils, force can be applied to adjust the position of the apparatus. Various other devices employ similar magnetic force actuators.
Similarly, U.S. Pat. No. 4,952,858 discloses a system for positioning a stage in a lithographic system using at least three magnetic coil actuators as well as at least three voice coil actuators. These actuators are mounted between the stage and a sub-stage, and together control the position of the stage in six degrees of freedom. Various other devices employ actuators between the stage and sub-stage, generally employing at least one actuator for each degree of freedom desired.
This disadvantages of these and other prior art systems include the difficulty in their assembly and operation, and the related possibility of errors during operation. These difficulties arise from, among other things, the various complexities associated with positioning and operating six or more force actuators between the stage and the sub-stage.